My 16lb dog gets very upset when the mail carrier delivers the mail. I think she hears the distinct engine sound of a US Mail truck. The mail trucks here in Little River, South Cackalacky, the driver gets to put a license plate of their choice on the the front. I am assuming they got to choose. So far I have seen a Grateful Dead one (a Steal Your Face), a USCG one, and a USN one as well.
So if I ever see - out for delivery, I know right when it gets dropped off, or put in the mail box.